1. Field of the Invention
The present invention may relate to a method for producing a fired decoration with an essentially defect-free visual appearance on substrates made of glass, glass-ceramic, or ceramic with other decorative colors consisting of base enamel, pigments, and additives.
2. Background Information
The substrates used can thereby be soda-lime glasses, borosilicate glasses, glass ceramics, porcelain, earthenware, stoneware, vitreous clay, and all substrates on which a decoration of ceramic colors can be applied, i.e., including the use of paper as a backing for transfer images.
Such a glass ceramic is described, for example, in European Patent No. 0 220 333 B1.
Ceramic colors are used in particular for the decoration in particular of glass and glass ceramics. These colors conventionally consist of one or more vitreous substances (base enamel or vitrified glass paste) and an admixture of one or more pigments (coloring agents).
Ceramic colors are conventionally produced by mixing frits (cracked glass paste) and the inorganic pigments. The mixing can thereby be done as early as during the process of melting the glass paste (fritting) or before or during a subsequent grinding process of the cracked glass paste. The ready-to-use ceramic colors are then available in the form of powder mixtures of pigments and glasses. The pigments, which may consist of metal oxides, are to a certain degree soluble in the molten glass. As a general rule, the solubility of the pigments increases with the temperature of the molten glass.
For decoration, in particular of glass and/or glass ceramic items, which are generally already in their final form, the powdered pigment is applied in a variety of ways to the substrate to be coated. The powder can be on average in particle diameters that range from less than approximately 1 micrometer up to approximately 40 micrometers.
In other words, the powder of the powdered pigment can be on average in particle diameters within a range from less than approximately 1 micron up to approximately 40 microns.
To fix the ceramic colors before firing, they are dispersed in essentially low-viscosity to essentially high-viscosity distribution-promotion agents, which volatilize partly during the drying of the applied colors and partly during the firing process. Examples of this step are the mixing with screen printing oil and the subsequent direct screen printing or indirect printing (e.g., via a transfer image) method, the mixing with thermoplastic material, and the subsequent screen printing with the application of heat.
In at least one possible embodiment of the present invention, screen-printable pastes may be produced using suspending media and solvents, for example, ethylcellulose, and screen-printing media, for example, fir oil-based media. The paste consistency set may determine the layer thickness of the decoration after firing.
To make the items to be decorated usable, the coatings applied to the substrate must be melted by firing them into a solid unit with the substrate. During heating, the glass powder preferably melts and forms a compact layer in which the pigment particles are embedded.
Such glass compounds for glazing, enameling and decoration, in particular of glass or glass ceramic materials, are known, for example, from German Patent No. 42 01 286 C2, German Patent Application No. DE-OS 42 41 411 C2, and German Patent No. 197 21 737 C1.
For the decoration of substrates, in particular of glass and glass ceramic objects, for example, by means of screen printing, the problem of reliably detecting flaws and defects in the decoration is very difficult. The thinner the decorative coating and the finer the pattern, the more difficult it is to see and detect defects before the decorative coating is fired. Consequently, after firing, the items on which the decoration is defective must be discarded, although otherwise they could be easily washed and reprinted before firing, which would significantly increase the yield of the production process.
During the decoration of glass ceramics, very thin, coarse coatings (approximately 8-10 micrometers) result, which have a relatively low contrast with the support e.g., in particular with a glass ceramic as disclosed in European Patent No. 0 220 333 B1.
One solution for this problem that has been used in the past was the addition of organic dyeing agents, which have long been used in the porcelain and ceramics industry, in particular for coloring ceramic compounds, e.g., Sudan(copyright), manufactured by BASF. Such additives are available in red, yellow, and blue colors and have the characteristic that they bum off essentially completely, leaving essentially no residue, at approximately 500 degrees Celsius, so that there are essentially no adverse effects on the finished product.
Some types of decorations, however, have patterns that are so extremely fine and thereby dense (e.g., dot diameter 0.5 millimeters) that the contrast, e.g., by means of dyeing with Sudan(copyright), is not sufficient to make it possible to reliably detect defects in the pattern before the piece is fired.
German Unexamined Patent Application No. DE-OS 44 22 338 A1 discloses hot-melt-type adhesives that contain a detectable content of at least one UV-active substance as a marking material. These UV-active substances are optical whitening agents, in particular thiophene-benzoxazol derivatives. German Patent Application No. DE-OS 40 24 871 A1 discloses styrene polymerizates that have antistatic properties and contain fluorescence coloring agents that are conventionally absorbent in the UV range, such as optical whitening agents of the type Uvitex(copyright)OB manufactured by CIBA, i.e., thiophene-benzoxazol derivatives.
German Patent No. 35 05 922 C1 relates to a method for the decoration of glass ceramic plates in which the decorative colors are applied before ceramicization and the ceramicization and the firing of the coloring substances are performed simultaneously.
German Patent No. 197 28 881 C1 describes glass ceramic articles decorated with ceramic colors.
None of the documents cited above discloses the use of a UV-sensitive optical whitening agent as an additive to a decorative color.
One object of the present invention may therefore be the development of a method to produce decorations which are essentially free of optical defects after they have been fired, and with which, even with extremely finely patterned decorations, it is possible to essentially easily and reliably detect variations from the ideal decoration before firing by means of a perceived color that becomes luminescent under UV radiation and which has a high contrast with the substrate.
In other words, in at least one possible embodiment of the present invention, an object of the present invention preferably is a method of producing decorations that, after they have been fired onto a substrate, are essentially free or completely free of defects when viewed with the naked human eye, even when the decorations have an extremely small, delicate, or fine pattern or have extremely small, delicate, or fine patterns. The detection of such defects, in at least one possible embodiment of the present invention, preferably is possible because of the use of a color, pigment, or coloring agent that may be perceived as a color by the naked human eye and that both preferably becomes luminescent under ultraviolet irradiation and preferably contrasts essentially highly with at least one coloring agent in the substrate on which the decoration is fired, or alternatively with at least one coloring agent in the decoration.
Additional objects of the invention may be the essentially complete absence of negative effects on the useful characteristics of the fired or ceramicized substrates as well as the essentially complete absence of toxicologically problematic reaction products or fission products, which objects may be achieved by the method.
These objects may be achieved by at least one possible object of the present invention, which preferably teaches that UV-sensitive optical whitening agents are added in the form of additives to the decorative color. In particular, thiophene-benzoxazol derivatives and products that contain 2,2xe2x80x2-(2,5-thiophenediyl)-bis(5-(1,1-dimethylethyl)-benzoxalate are added to the decorative color as optical whitening agents. The additives can thereby be added by being mixed into the screen printing oil, for example, or by stirring into the coloring substance that has been processed into a paste form.
At least one possible embodiment of the present invention preferably teaches that the additive is added to and homogeneously mixed with the decorative color in finely divided powder form in quantities of approximately 0.05 to approximately 10 percent by weight, based on the total weight of the composition, in particular in quantities of approximately 0.1 to approximately 1 percent by weight, based on the total weight of the composition.
At least one possible embodiment of the present invention preferably teaches that it is particularly advantageous if the additives have UV-reflecting, fluorescent characteristics. Under UV light, it is often much easier to detect flaws and defects, either with the naked eye or with appropriate measuring instruments, because the contrast with the background may be significantly greater. The range of radiation most suitable for an optimal increase in contrast can thereby be determined as a function of the substrate and the decorative color in question by means of preliminary tests on a case-by-case basis.
At least one possible embodiment of the present invention preferably teaches that decorative colors suitable for screen printing are preferably used in the method, because these colors may meet the highest quality requirements for the configuration and realization of the decorations.
The decorations may be used, for example, for inscriptions, for achieving a desired design, or to support certain technical functions, for example, for display windows or for marking cooking zones.
The glaze, enamel, or decoration may be fired at temperatures below the softening point of the article to be coated, during which the respective glass composition of the glaze or enamel melts and forms a stable bond to the surface of the article. The firing temperatures are generally below the softening point of the article to be coated in order that uncontrolled deformation cannot occur. The firing also serves to volatize organic auxiliaries used, for example, as suspension media for applications of the glaze, enamel, or decoration.
The above-discussed embodiments of the present invention will be described further hereinbelow. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is, the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintain that this application may include more than one patentably and non-obviously distinct invention. Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.